The present invention relates to high voltage transistors, and more particularly, to a transistor having low on-resistance using a spiral field plate and graded concentration doping in the drift region.
Integrated high voltage transistors have found great application in power and switching devices. The most common high voltage transistor is the N-channel lateral MOSFET using reduced surface field (RESURF). In this device, the dopant concentration in the drift region is optimized at 1xc3x971012/cm2. See, e.g., xe2x80x9cHigh Voltage Thin Layer Devices (RESURF Devices),xe2x80x9d Appels and Vaes, Philips Research Laboratories, IEDM 1979. However, this device has an on-resistance that is very much limited by the breakdown voltage.
Improvement may be obtained by optimizing the dopant concentration throughout the drift region. For example, instead of maintaining a constant doping throughout the drift region, a two-zone structure may be used. This was demonstrated by Yamaguchi et al. as detailed in xe2x80x9cProcess and Design of a 1000 volt MOS IC,xe2x80x9d IEDM 1981, Yamaguchi and Morimoto. This structure can obtain a slightly higher average concentration of dopant so that the overall drift region resistance is slightly lower. Lowering the concentration in the drift region near the channel can prevent punch-through of the channel by allowing the depletion region to expand more into the more lightly doped drift region. This results in lower on-resistance because the drift region near the drain contact may be more heavily doped.
A second disadvantage of the RESURF lateral transistor is that high current density operation induces low breakdown voltage. See xe2x80x9cKirk Effect Limitations in High Voltage ICs,xe2x80x9d ISPSD 1994. During high voltage switching transients, high current densities can occur in the drift region. High current density causes distortion of the field potential due to an excess of majority carriers injected from the source into the drift region. The majority carriers injected from the source disturb the space charge equilibrium established in the off state. This causes the space charge width (depletion width) to decrease. With the high potential difference across a narrowing distance, the electric field must increase. If the injected current density is sufficiently high, the distortion of the space charge region can cause the electric field to exceed the critical field in silicon and breakdown will occur.
Yet another disadvantage of the RESURF device is its long-term reliability. See, for example, xe2x80x9cOptimization and Surface Charge Sensitivity of High Voltage Blocking Structure with Shallow Junctions,xe2x80x9d IEEE Transactions on Electronic Devices, Vol. 38, No. 7, July 1991. Since the drift region surface is exposed to the mobile ions in the interlayer dielectric, the surface potential can change drastically due to dielectric film variations and the application of long-term high voltage bias. When the surface electrical field is distorted, the breakdown voltage may change.